What is Iran’s Nuclear Red Line?

Tehran’s denials and protestations to the contrary, its nuclear ambitions clearly go beyond peaceful, civilian purposes. The International Atomic Energy Agency (IAEA) is soon expected to issue a report stating that Iran has increased its capacity to enrich uranium but is limiting the most worrisome activity. This raises the question of how far Iran wishes to proceed down the nuclear path. The answer is important, as there is an important distinction between an Iran that has assembled (and perhaps tested) a nuclear weapon, and an Iran that has a latent nuclear capability but does not take the final step of pulling the pieces together to have a nuclear weapon.

The short answer to the question now is that we do not know. The U.S. intelligence community assesses that Iran wishes to have a nuclear weapons option but has not yet decided whether to build a bomb.

Concern about Iran’s nuclear intentions has grown as it has enriched uranium at facilities at Natanz and Fordow, facilities about which Tehran did not inform the IAEA until others revealed them. The Iranians conduct uranium enrichment operations to 3.5 percent, which they say they need for fuel rods for nuclear power reactors, despite the fact that Russia has contracted to sell Iran the fuel rods that it needs for its sole power reactor at Bushehr. More troublesome, Iran also enriches to 20 percent. The Iranian government claims that it needs 20 percent enriched uranium for fuel for the Tehran research reactor, though it is not clear that Iran has the technical capability to produce fuel rods for that reactor.

Although 20 percent qualifies as “highly-enriched uranium,” weapons-grade uranium is enriched to 90 percent or greater. Once uranium has been enriched to 20 percent, it is much of the way to 90 percent. Interestingly, while enriching to 20 percent, Iran has taken some of the resulting uranium gas (referred to as uranium hexafluoride) and converted it to uranium oxide, a solid powder. Iran thus has kept its stock of uranium hexafluoride enriched to 20 percent below the amount that, if enriched to 90 percent, would suffice for a nuclear bomb. Some see that as a signal that Tehran is sensitive to Western concerns. While the process of converting uranium hexafluoride to uranium oxide can be reversed, it takes time.

Of course, Iran must do more than have fissile material. Can it construct a deliverable nuclear weapon? Building a “gun-type” bomb is relatively simple (to the extent that the physics of nuclear weapons can be called simple). U.S. scientists in 1945 were so confident of the design for the bomb used on Hiroshima that they did not bother to test it. But a gun-type weapon would be large and bulky, probably weighing on the order of five tons.

Building a sophisticated weapon that could fit in a ballistic missile warhead—the delivery means of choice—poses a more demanding technical task. The weapon needs to be small and durable enough to withstand the dynamic and thermal stresses of ballistic flight. While the IAEA has questions about past Iranian weaponization activities, the U.S. intelligence community has assessed that Iran in 2003 halted its nuclear weapons program (PDF), which it defined as weaponization work as well as enrichment.

As for the delivery system, Iran has an inventory of short- and medium-range ballistic missiles. The Sajjil-2, currently under development, has an estimated range of 2200 kilometers. That puts the Gulf states, Israel and southeastern Europe in range, but Iran still has a long way to go before it could develop an intercontinental ballistic missile capable of reaching the United States.

So Iran has made varying degrees of progress down the tracks—enrichment, weaponization and delivery system—needed to have a viable nuclear weapon. How far will it proceed?

One option is to build a bomb and, to show the world its nuclear prowess, conduct a test. But that option poses real risks for the Iranian government. It would make Iran even more of a nuclear pariah and increase its international isolation. It would provoke huge concern in Saudi Arabia and the Gulf states, perhaps leading the Saudis—and others such as Egypt and Turkey—to pursue their own nuclear weapons programs. And it would indisputably cross the red lines that Jerusalem and Washington have drawn.

The second option is that Iran develops its enrichment, weaponization and missile technologies so that it has a latent nuclear weapons capability but stops short of putting the pieces together. In this option, Tehran might continue to limit its stock of uranium hexafluoride enriched to 20 percent by converting some to uranium oxide. Assuming that Iran does not have a covert enrichment facility (something Western intelligence services undoubtedly spend considerable time and effort looking for), we would know of an Iranian decision to enrich its uranium to weapons-grade, as the IAEA monitors its facilities at Natanz and Fordow. While experts differ regarding how much alert time the world would have, there would be tactical warning—currently measured in months—of a decision by Tehran to produce weapons-grade uranium.

The distinction between these two options is important. While no one, particularly Israel, would be comfortable with a latent Iranian nuclear capability, that is vastly preferable to an Iran with even a small stockpile of nuclear weapons. It would pose less of a threat to the nuclear non-proliferation regime. It would leave time for international sanctions to intensify their impact on the Iranian economy and perhaps affect the calculations in Tehran. And it would give the UN Security Council Permanent Five plus Germany time to explore whether the Iranian government is prepared to consider a negotiated settlement that satisfies international concerns.